Refactor: Migrate from Retrofit to Ktor

This commit migrates the network layer from Retrofit to Ktor.

Specific changes include:

- Replaced Retrofit
 with Ktor for network requests.
- Updated dependencies to include Ktor libraries.
- Refactored network service and data classes to use Ktor's API.
- Removed Retrofit-specific code and dependencies.
- Adjusted network module to provide Ktor client and services.
- Updated PixabayImageRepository
 to use the new Ktor-based PixabayService.

app/src/main/java/dev/adriankuta/pixabay/PixabayNavGraph.kt

@@ -43,7 +43,6 @@ fun PixabayNavGraph(
         ) { entry ->
             PhotoDetailRoute(
                 photoId = entry.arguments?.getInt(PHOTO_ID_ARG)!!,
-                onBack = { navController.popBackStack() },
             )
         }
     }